Transforming growth factors ("TGFs") are polypeptides that, in certain nontumorigenic cell types, reversibly induce features resembling those of malignant transformation. Two distinct types of TGFs are known--TGF-.alpha. and TGF-.beta.. TGF-.alpha. is related to epidermal growth factor (EGF) and binds to the same cellular receptors as EGF.
TGF-.beta.s, which have been found in a variety of cells, including T-lymphocytes, monocytes and platelets, are structurally and antigenically distinct from TGF-.alpha. and comprise a group of closely related paracrine factors that elicit a variety of biological effects. Cellular responses to TGF-.beta.s may be grouped into three main categories: proliferative responses; effects on cell differentiation and differentiated functions; and responses involving cell adhesion, migration and extracellular matrices.
In most cases, TGF-.beta.s inhibit cellular proliferation. TGF-.beta.s are also known to regulate differentiation of myoblasts, osteoblasts, chondroblasts, preadipocytes, hematopoietic progenitors and other cells. Depending on the type or lineage of specific cells, such regulation may either favor or reversibly inhibit differentiation. For example, TGF-.beta.s can regulate the specialized functions of lymphocytes and hormone production by adrenocortical, granulosa and pituitary cells.
TGF-.beta.s can also produce alterations in the extracellular matrix or autocrine cytokine production that may play a role in the physiological action of these polypeptides in tissue morphogenesis, remodelling and repair, including wound healing. See, e.g., U.S. Pat. No. 5,104,977. Moreover, TGF-.beta. are known to stimulate fibronectin and. collagen production by dermal fibroblasts in vitro.
The in vivo administration of TGF-.beta. to newborn mice was shown to induce a rapid fibrotic response and accumulation of fibroblasts into the injection site. More recently, platelet-derived TGF-.beta. was shown to be a potent chemoattractant for fibroblasts in vitro. (Postlewaite, A. et al., J. Exptl. Med. 165:251-256, 1987). The ability of TGF-.beta. to be a chemoattractant for fibroblasts and inflammatory cells, e.g., neutrophils and monocytes/macrophages has led to the finding that this cytokine apparently plays important roles in wound healing and tissue repair. (See Davidson, J. M., in Inflammation: Basic Principals and Clinical Correlates, Second Edition, Gallin, Goldstein and Snyderman, eds., Raven Press, 11992, pp. 809-819, for review). For example, PCT WO 84/001106, filed Sep. 23, 1983, describes TGF-.beta.1 and its use in promoting cell proliferation, tissue repair, wound healing and treatment of trauma. See also U.S. Pat. No. 5,104,977.
Structurally, TGF-.beta.s are disulfide-linked 25 kDa dimeric proteins, with the mature monomeric polypeptide comprising 112 amino acids. The mature chain is derived from a larger precursor comprising approximately 390-412 amino acids (Derynck et al., Nature 316:710-705, 1985; Massague, J. et al., Cancer Surveys Volume 12: Tumor Suppressor Genes, the Cell Cycle and Cancer, pp. 81-103, 1992). The biologically active mature polypeptide chain is derived from the carboxyterminus of the precursor. Apparently, proteolytic cleavage to the mature protein occurs intracellularly, after dimerization of the precursor.
There are at least five known isoforms of TGF-.beta. (designated TGF-.beta.1-5), which are about 70% homologous in amino acid sequence to each other. At least three genes (apparently located on different chromosomes) encode the TGF-.beta.1, .beta.2 and .beta.3 precursors in the human and other mammalian genomes. Two additional TGF genes, coding for TGF-.beta.4 and TGF-.beta.5, have been identified in chicken and Xenopus laevis genomes, respectively. The degree of amino acid sequence homology between .beta.1, .beta.2 and .beta.3 polypeptide chains is about 70% (range 64-82%). On the other hand, the amino acid sequence of each TGF-.beta. isoform is strictly conserved, with the sequence homology of, e.g., .beta.1 being nearly 100% between the chicken and human polypeptides (similar homologies have been observed for .beta.2 and .beta.3 between chicken and human).
Generally, the constituent polypeptide chains form homodimers to yield TGF-.beta.1, .beta.2 and .beta.3 proteins, respectively. Heterodimers, however, may be formed in some cells which simultaneously express different TGF-.beta. genes, e.g., porcine megakaryocytes, to yield platelets containing TGF-.beta.1.2 heterodimers, in addition to homodimers.
As discussed above, TGF-.beta.1 has proved to be a potent chemotactic agent for cells of mesenchymal origin, e.g., fibroblasts and inflammatory cells, including neutrophils and monocytes/macrophages. In large part because of its activity as a chemoattractant, TGF-.beta.1 is believed to play a critical role in inflammatory and tissue repair processes, including wound healing. The recruitment of inflammatory cells to the site of a wound is necessary for proper tissue repair. Likewise, the recruitment and subsequent proliferation of fibroblasts, which initiate a fibrogenic response, are also required. Prior to the present work, however, it was unknown what structural features of TGF-.beta.1 (or the other TGF-.beta.s) would be involved in the ability of the peptide to cause chemotaxis of cells. Accordingly, the present invention provides novel peptides corresponding to the amino acid sequence of TGF-.beta.s that are chemotactic for fibroblasts and inflammatory cells and may be used in wound healing and tissue repair compositions.